1. Field of the Invention
The invention relates generally to laser systems comprised of a master oscillator and a power amplifier. The invention relates particularly to the use of neodymium-doped crystals or glasses in the oscillator and to ytterbium-doped materials in the amplifier. The invention relates most specifically to laser systems operating at 1.047 .mu.m, where Nd:LiYF.sub.4 oscillators (Nd:YLF) are employed in conjunction with diode-pumped Yb:Sr.sub.5 (PO.sub.4).sub.3 F amplifiers (Yb:S-FAP).
2. Description of Related Art
One of the most common architectures employed for laser systems consists of the master oscillator (MO) and power amplifier (PA) or MOPA. Here, the roles of initially generating a laser light beam with certain characteristics, and then amplifying this light beam up to a particular power level, are separated in the laser system. This architecture allows the designer to independently optimize the generation and amplification processes.
The purpose of the master oscillator (MO) is most commonly to generate a light beam that meets certain specifications related, for example, to the wavelength, beam quality, spectral width, temporal format, and stability. The MO often includes additional optical components aside from the customary mirrors and gain element, such as a Q-switch, etalon, Bragg cell, and/or aperture. The extra optical components manipulate the light beam in order to impose the desired characteristics on it. The oscillator gain element is most suitably capable of having a low threshold of operation and of inducing minimal optical and thermal distortions to the laser beam. The main idea is that the MO is a low power or energy device, although it is precise in realizing certain beam parameters.
In contrast, the predominating concerns surrounding the power amplifier (PA) design commonly relate to cost and efficiency. In other words, the MO is envisioned to provide a relatively weak beam with certain precisely tailored properties, while it is the purpose of the PA to greatly increase its power and energy. The PA generally requires greater electrical input and utilizes larger optical gain elements than the MO.
The present invention relates mainly to amplifiers that are pumped by quasi-coherent sources (e.g. lasers, diodes, or laser diode technology), rather than flashlamps. As a consequence of the relatively large cost associated with laser diode pump sources per unit of light power, the use of gain media that can provide greater pumping efficiency will essentially serve to reduce the diode costs. In this way, the employment of superior gain media in the amplifier will optimize both the cost and efficiency of the amplifier. One key requirement of the MOPA is that the operating wavelength of the MO must fall within the useful gain bandwidth of the PA.
The MOPA architecture is usually implemented by utilizing the same gain medium throughout the laser. For example, most commercial Nd:YAG (neodymium-doped Y.sub.3 Al.sub.5 O.sub.12) lasers utilize a flashlamp-pumped oscillator and amplifier. The oscillator is often arranged to operate mode-locked, Q-switched, or with a single longitudinal mode; the power amplifier is sometimes designed with a larger laser rod and/or additional flashlamps. Other common laser materials such as Nd:YLF, Ti:sapphire, and alexandrite are routinely deployed in systems using the standard MOPA architecture. The obvious advantage of this strategy is that the oscillator and amplifier are guaranteed to operate at the same wavelength.
An early example of a hybrid MOPA laser system (using dissimilar gain media) is the Nd:YAG oscillator used in conjunction with an amplifier based on Nd-doped silicate glass (Laser Program Annual Report--1979, UCRL-50021-79, available from National Technical Information Service, Springfield, Va). A special advantage of this combination is related to the capability of fabricating Nd:glass into large optical elements &gt;100 cm.sup.3 (to be compared to the modest size possible with Nd:YAG of .about.10 cm.sup.3). This aspect of Nd:glass permits the design of large, cost effective amplifiers. The Nd:YAG crystal, on the other hand, is especially amenable to oscillator design, due in part to its high thermal conductivity and large gain cross section. For these reasons, Nd:YAG oscillators offer both low laser threshold operation and straightforward means of thermal management. Although Nd:YAG does not absorb flashlamp light effectively, this issue is not considered crucial since the efficiency of the oscillator in a MOPA system is not central. Since the total energy associated with the MO is small compared to the PA, an inefficient MO does not appreciably impact the overall MOPA system efficiency. This example is illustrative of the concept that MOPA laser system performance can be optimized with different gain media (albeit the same Nd laser ion) in the oscillator and amplifier.
Another similar example is the hybrid MOPA system utilizing Nd:YLF and Nd-doped phosphate glass (C. Bibeau, D. R. Speck, R. B. Ehrlich et al., "Power, energy, and temporal performance of the Nova Laser facility with recent improvements to the amplifier system," Applied Optics 31, 5799-5809 (1992)). Both this example and the previous one entail using the same laser ion in the oscillator and amplifier (i.e., neodymium). In this way, the oscillator and amplifier are likely to have sufficient coincidental matches of the peak gain wavelength, since the Nd.sup.3+ ion typically lases within a relatively small range in most host materials (.about.1.04-1.09 .mu.m).
A few hybrid MOPA systems employ materials based on different laser ions in the oscillator and amplifier. For example, Ti:sapphire oscillators can be used in conjunction with several types of amplifiers, since this laser material can operate between 0.66-1.1 .mu.m (P. F. Moulton, "Spectroscopic and laser characteristics of Ti:Al.sub.2 O.sub.3," Journal of Optical Society of America B 3 125-133 (1986)). Ti:sapphire lasers can also be self-modelocked to generate pulses of .about.100 femtosecond duration. Flashlamp-pumped 0.83 .mu.m Cr:LiSAF (T. Ditmire and M. D. Perry, "Terawatt Cr:LiSrAlF.sub.6 laser system," Optics Letters 18, 426-428 (1993)) and 1.05 .mu.m Nd:glass amplifiers can be configured to amplify these pulses to a much higher level. In either case, the amplifiers are a more efficient and cost-effective means of greatly amplifying the oscillator output compared to Ti:sapphire.